With the expected depletion of fossil fuel resources such as petroleum or coal available as industrial raw materials, increasing attention has been focused on permanently usable biomass. Of biomass, lignocellulosic biomass is highly useful because there is no other competing use such as that of food resources. Lignocellulosic biomass is composed of carbohydrates such as cellulose and lignin. Lignin is a natural phenolic polymer which accounts for 15-20% of lignocellulosic biomass.
Rigid polyurethane foams have good insulation characteristics and flame retardant characteristics, and thus are widely used in insulating materials for refrigerators, freezers, and other conventional buildings, as well as in insulating panels and the like. In general, a rigid polyurethane foam may be prepared by reaction of a polyol and an isocyanate in the presence of a catalyst.
Recently, there have been attempts to use biomass-originating lignin in preparation of rigid polyurethane foams. For example, lignosulfonate obtained by a sulfite pulping process, alkali lignin obtained by a soda pulp process, a kraft lignin from kraft paper manufacturing process, and the like may be used.
However, these lignins do not dissolve easily in polyols and cause excessive cross-linking, which deteriorates physical properties in rigid polyurethane foams. To address these problems, a hydroxyl value of lignin may be reduced through chemical modification such as acetylation or esterification, so that lignin can be used in preparation of polyurethane. Therefore, there is a need for rigid polyurethane foams which may be prepared by partially substituting a petroleum-originating polyol with lignin without additional chemical modification.